Unravelling the Origins of Hydroboration Chemoselectivity Inversion Using an N,O-Chelated Ir(I) Complex: A...

It has been widely reported that the transition-metal catalysts can invert the chemoselectivity of hydroboration of multifunctional substrates, but the fundamental reasons for this inversion have rarely been studied. In this work, mechanistic details and chemoselectivity of the hydroboration of various E-C ( E = CH, CH₂, O, N, etc.) multiple bonds has been explored through density functional theory calculations, including reactions using free HBCy₂ or captured HBCy₂ by an N,O-chelated Ir(I) complex which includes a phosphoamidate group. The computational results demonstrate the formation of a 3-center π complex before the 4-center transition state when using free HBCy₂. When the Ir(I) complex is added, the HBCy₂ would be captured to form a metallaheterocyclic Shimoi-type intermediate in which a δ-[M]···H-B agostic interaction appears. Afterward, the E-C functional group intersects this hemilabile interaction rather than the P═O···B coordination bond, and the following hydride transfer is found to be rate-determining. The frontier molecular orbital analysis discloses that the olefins that have typical delocalized π electrons tend to give more effective overlap with the LUMO of free HBCy₂, but the aromatic substituents could significantly reduce its reactivity due to the enhanced electron delocalization. The polarized carbonyl group facilitates σ coordination with the metal center, but the vinyl group favors interaction through π coordination. The superiority of σ coordination results in the priority of hydroboration of aldehyde over the olefins when using the captured HBCy₂. Thus, it is the conversion of the more favored bonding groups by the transition metal that should be fundamentally responsible for the chemoselectivity inversion.